Porous insulating film and its laminates

ABSTRACT

A porous insulating film comprising a highly heat resistant resin film having a fine porous structure with a mean pore size of 0.01-5 μm in at least the center of the film, and a porosity of 15-80%. A laminate is prepared by forming a heat resistant adhesive layer or a conductive metal layer or an inorganic or metal substrate on one or both sides of the porous insulating film or by forming an inorganic or metal substrate on one side of the porous insulating film and a conductive metal layer on the other side.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a porous insulating material andits laminates, that exhibit excellent electrical properties (lowdielectric constant) and heat resistance, and particularly it relates toa porous insulating material and its laminates that are resistant todielectric deterioration by dielectric breakdown or the like and arestrongly adhesive or adhesive, and that are highly useful forhigh-frequency electronic parts, and especially distribution substrates.

[0003] Throughout the present specification, “insulating film” willrefer to an electrical insulating film. “Fine continuous pores” willrefer to fine pores that form continuous channels from one side toanother, where the fine pores preferably run in a nonlinear fashion fromone side to the other on a nonlinear path.

[0004] 2. Description of the Related Art

[0005] With the rapid increase. in communication data in recent yearsthere has been an increasing desire for miniaturization, weightreduction and higher speeds for communication devices, which has led toa demand for suitable low-dielectric constant electrical insulatingmaterials. In particular, the frequency band ranges of the waves usedfor portable mobile communications such as automobile phones, digitalcellular phones and satellite communications are the high-frequencybands in the MHz and GHz ranges. In the rapid development ofcommunication devices used as the means for such communication, designshave been produced for mounting small high-density boards, substratesand electronic elements. For miniaturization and weight reduction ofcommunication devices suited for high-frequency ranges such as the MHzto GHz bands, it is necessary to develop electric insulating materialshaving both excellent high-frequency propagation characteristics andappropriate low dielectric constant characteristics.

[0006] Specifically, energy loss occurs in circuit elements during thecourse of propagation, and is known as dielectric loss. This energy lossis undesirable because it is expended as heat energy in the circuitelement and is released as heat. The energy loss is produced in lowfrequency ranges by variation in the electric field of the dipolecreated by dielectric polarization, and is produced in high frequencyranges by ionic polarization and electronic polarization. The ratio ofthe energy expended in a dielectric material and the energy stored inthe dielectric material in one cycle of an alternating electric field iscalled the dielectric loss tangent, and it is represented by tanδ. Thedielectric loss is proportional to the product of the dielectricconstant ε and the dielectric loss tangent of the material. Thus, atotal energy loss due to tanδ is increased in the higher frequencyrange. Also, since the heat release per unit area is larger whenmounting high-density electronic elements, a material with a small tanδmust be used to suppress dielectric loss of the insulating material. Byusing a low dielectric polymer material with low dielectric loss, theheat release due to dielectric loss and electrical resistance iscontrolled, thus helping to reduce signal malfunction; materials withlow transmission loss (energy loss) are therefore in great demand in thefield of high frequency communications.

[0007] Various materials having such electrical properties includingelectrical resistance characteristics and low dielectric constant havebeen proposed, among which are thermoplastic resins such as polyolefins,vinyl chloride resins and fluorine-based resins, and thermosettingresins such as unsaturated polyester resins, polyimide resins, epoxyresins, vinyltriazine resins (VT resins), crosslinked polyphenyleneoxide, curable polyphenylene ethers, and the like.

[0008] Polymers containing fluorine atoms in the molecular chain, suchas vinylidene fluoride resins, trifluoroethylene resins andperfluoroethylene resins, have excellent electrical characteristics (lowdielectric constant, low dielectric loss), heat resistance and chemicalstability, but unlike thermoplastic resins they are poorly suited forshape forming and coat forming to obtain molds or films through heattreatment, and they increase costs when used for device manufacture. Inaddition, their low transparency is a drawback that limits the range ofapplicable fields. Since the above-mentioned low dielectric constantgeneral use polymer materials all have allowable maximum temperatures ofbelow 130° C., as electrical instrument insulating materials their heatresistance classification is type B or below as specified by JIS-C4003,and therefore their heat resistance is inadequate.

[0009] As resins with relatively good heat resistance there may bementioned thermosetting resins such as epoxy resins, polyphenylene ether(PPE), unsaturated polyester resins and phenol resins. However, none ofthese exhibit a satisfactory level of heat resistance and dielectricconstant.

[0010] Further performance demanded for low dielectric constantmaterials with excellent dielectric/insulating resistancecharacteristics often includes a requirement for soldering resistancethat can withstand heating at 260° C. or above for at least 120 secondsbecause of the soldering step in the device manufacturing process.

BRIEF SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a porousinsulating material with heat resistance, low dielectric constant anddielectric loss and excellent insulating properties, as well aslaminates thereof.

[0012] The present invention relates to a porous insulating filmcomprising a highly heat resistant resin film having a fine porousstructure with a mean pore size of 0.01-5 μm in at least the center ofthe film, and having a porosity of 15-80%.

[0013] The invention further relates to a laminate prepared bylaminating a heat resistant adhesive layer on one or both sides of theaforementioned porous insulating material.

[0014] The invention still further relates to a laminate prepared bylaminating a conductive metal layer for an electronic circuit on one orboth sides of the aforementioned porous insulating material, eitherdirectly or via a heat resistant adhesive layer.

[0015] The invention yet further relates to a laminate prepared bylaminating an inorganic or metal substrate onto one or both sides of theaforementioned porous insulating film and a conductive metal layer ontothe other side, each via a heat resistant adhesive layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0016]FIG. 1 is an enlarged cross-sectional schematic drawing of aporous polyimide film as a porous insulating material according to anembodiment of the invention.

[0017]FIG. 2 is an enlarged cross-sectional schematic drawing of alaminate formed by laminating two layers of a porous polyimide film as aporous insulating material according to an embodiment of the invention.

[0018]FIG. 3 is an enlarged cross-sectional schematic drawing of a oneside-densified porous polyimide film according to an embodiment of theinvention.

[0019]FIG. 4 is an enlarged cross-sectional schematic drawing of a bothside-densified porous polyimide film according to an embodiment of theinvention.

[0020]FIG. 5 is an enlarged cross-sectional schematic drawing of alaminate formed by laminating a porous polyimide film as a porousinsulating material on an inorganic, organic or metal substrate,according to an embodiment of the invention.

[0021]FIG. 6 is an enlarged cross-sectional schematic drawing of alaminate wherein an inorganic, organic or metal substrate such as asilicon substrate is formed on one side of a porous polyimide film as aporous insulating material according to an embodiment of the inventionvia a heat resistant adhesive, while a dense polyimide layer, andadditionally a conductive metal circuit layer, are formed on the otherside.

[0022]FIG. 7 is a scanning electron microscope photograph of the surfaceof a porous polyimide film as the porous insulating material obtained inExample 1.

[0023]FIG. 8 is a scanning electron microscope photograph of across-section of a porous polyimide film as the porous insulatingmaterial obtained in Example 1.

[0024]FIG. 9 is a scanning electron microscope photograph of across-section of a both side-densified porous polyimide film accordingto an embodiment of the invention.

[0025] In FIGS. 1-6, 1 is a porous polyimide film as a porous insulatingmaterial, 2 represents continuous pores, 3 is a highly heat resistantresin, 4 is a dense layer, 5 is an inorganic, organic or metalsubstrate, such as a silicon substrate, 6 is a heat resistant adhesive,7 is a conductive metal. circuit layer, 10 is a laminate and 11 isanother laminated porous polyimide layer as a porous insulatingmaterial.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The embodiments of the invention are as follows.

[0027] 1) The aforementioned porous insulating film wherein the fineporous structure comprises fine continuous pores.

[0028] 2) The aforementioned porous insulating film which is fabricatedby film casting.

[0029] 3) The aforementioned porous insulating film which has adielectric constant of no greater than 2.5.

[0030] 4) The aforementioned porous insulating film wherein the highlyheat resistant resin film is a polyimide film.

[0031] 5) The aforementioned porous insulating film wherein the porousstructure contains fine continuous pores reaching to both surfaces.

[0032] 6) The aforementioned porous insulating film wherein the porousstructure includes a dense layer on both surfaces of the film.

[0033] 7) The aforementioned porous insulating film which has a porosityof 30-80%, a maximum pore size of no greater than 10 μm, a filmthickness of 5-100 μm, a resistance to passage of air of from 30 sec/100cc to 2000 sec/100 cc, a heat resistance temperature of 200° C. or aboveand a heat shrinkage of no greater than ±1%, and which has finecontinuous pores reaching to both surfaces.

[0034] The highly heat resistant resin according to the invention may bea resin made of a high molecular weight and highly heat resistantpolymer produced by condensation polymerization and heating of an acidcomponent and a diamine component, and it is preferably an aromaticpolyimide.

[0035] The following explanation will deal with a case using an aromaticpolyimide as the highly heat resistant resin.

[0036] The present invention will now be explained with reference to theattached drawings.

[0037]FIG. 1 is an enlarged cross-sectional schematic drawing of aporous polyimide film as a porous insulating film (hereunder alsoreferred to as “porous insulating material”) according to an embodimentof the invention.

[0038]FIG. 2 is an enlarged cross-sectional schematic drawing of alaminate (substrate) formed by laminating two layers of a porouspolyimide film as a porous insulating film according to an embodiment ofthe invention.

[0039]FIG. 3 is an enlarged cross-sectional schematic drawing of aporous polyimide film with a dense layer on one side according to anembodiment of the invention.

[0040]FIG. 4 is an enlarged cross-sectional schematic drawing of aporous polyimide film with a dense layer on each side according to anembodiment of the invention.

[0041]FIG. 5 is an enlarged cross-sectional schematic drawing of alaminate (substrate) formed by laminating a porous polyimide film as aporous insulating film on a silicon substrate, according to anembodiment of the invention.

[0042]FIG. 6 is an enlarged cross-sectional schematic drawing of alaminate wherein an inorganic, organic or metal substrate such as asilicon substrate is formed on one side of a porous polyimide film as aporous insulating material according to an embodiment of the inventionvia a heat resistant adhesive, while a dense polyimide layer, andadditionally a conductive metal circuit layer, are formed on the otherside.

[0043] The formation of more than two different substrates in FIGS. 2 to5 can be accomplished, for example, by laminating a precursor porousfilm on a substrate of the same or a different type by applying pressureif necessary, and then heating and drying it.

[0044] The porous polyimide film used as a representative example of aporous insulating material of the invention may be fabricated, forexample, by any of the following three film casting methods. Filmcasting can give porous insulating films suitable as porous electricinsulating materials having uniform pore sizes and satisfactory surfacesmoothness even with thin film thicknesses.

[0045] As a first method, there may be mentioned a method in which apolyimide precursor solution comprising 0.3-60 wt % of a polyimideprecursor and 99.7-40 wt % of a solvent is cast into a film andcontacted with a solidifying solvent with a solvent substitution rateadjustor to precipitate the porous polyimide precursor, and then thepore-formed polyimide precursor film with fine continuous pores reachingto both surfaces is subjected to thermal imidation or chemical imidationto obtain a porous polyimide film.

[0046] As a second method, there may be mentioned a method in which apolyimide precursor solution comprising (A) a homogeneous solution of apolyimide precursor and (B) a poor solvent for the polyimide precursoris cast into a film and the film-like composition is heat treated forimidation to obtain a porous polyimide film having a dense layer,preferably of a thickness of 0.2-5 μm, on both sides of the film and afine porous layer at the center of the film.

[0047] As a third method, there may be mentioned a method in which apolyimide precursor solution comprising 0.3-60 wt % of a polyimideprecursor and 99.7-40 wt % of a solvent is cast into a film andsubjected to treatment by exposure to vapor of a non-solvent for thepolyimide precursor, after which it is contacted with a solidifyingsolvent to form pores and then the porous polyimide precursor film withfine continuous pores reaching to both surfaces is subjected to thermalimidation or chemical imidation to obtain a porous polyimide film.

[0048] The polyimide precursor is a polyamic acid or a partiallyimidated form thereof, obtained by polymerization of a tetracarboxylicacid component and a diamine component, preferably an aromatic monomer,and it may be made into a polyimide resin with ring closure by thermalimidation or chemical imidation. The polyimide resin is a heat resistantpolymer wherein the imidation rate explained hereunder is about 80% orgreater, and preferably about 95% or greater.

[0049] The organic solvent (good solvent) used as the solvent for thepolyimide precursor may be parachlorophenol, N-methyl-2-pyrrolidone(NMP), pyridine, N,N-dimethylacetamide, N,N-dimethylformamide,dimethylsulfoxide, tetramethylurea, phenol, cresol, or the like.

[0050] The tetracarboxylic acid component and the aromatic diaminecomponent is dissolved in the above-mentioned organic solvent in roughlyequimolar amounts for polymerization, producing a polyimide precursorwith an inherent viscosity (30° C., concentration: 0.5 g/100 mL NMP) ofat least 0.3, and especially 0.5-7. When the polymerization is carriedout at a temperature of about 80° C. or above, a polyimide precursor isproduced which is imidated by partial ring closure.

[0051] The aromatic diamine is preferably an aromatic diamine compoundrepresented, for example, by general formula (1),

H₂N—R(R¹)_(m)-A-(R²)_(n) ^(R)′—NH₂  (1)

[0052] where R and R′ are direct bonds or divalent aromatic rings and atleast one is a divalent aromatic ring, R¹ and R² are substituents suchas hydrogen, lower alkyl, lower alkoxy, halogen, etc., A is a directbond or a divalent group such as O, S, CO, SO₂, SO, CH₂, C(CH₃)₂, etc.and m and n are integers of 1-4.

[0053] As specific aromatic diamine compounds there may be mentioned4,4′-diaminodiphenylether (hereunder abbreviated as DADE),3,3′-dimethyl-4,4′-diaminodiphenylether,3,3′-diethoxy-4,4′-diaminodiphenylether and paraphenylenediamine(hereunder abbreviated as p-PDA), with 4,4′-diaminodiphenylether andparaphenylenediamine being preferred.

[0054] The aromatic diamine component may also be a diaminopyridine, andspecifically there may be mentioned 2,6-diaminopyridine,3,6-diaminopyridine, 2,5-diaminopyridine and 3,4-diaminopyridine.

[0055] The aromatic diamine component may also comprise two or more ofthe aforementioned aromatic diamines in combination.

[0056] The tetracarboxylic acid component is preferably abiphenyltetracarboxylic acid component, with preferred examplesincluding 3,3′,4,4′-biphenyltetracarboxylic dianhydride (hereunder alsoabbreviated as s-BPDA) and 2,3,3′,4′-biphenyltetracarboxylic dianhydride(hereunder also abbreviated as a-BPDA), but it may also be 2,3,3′,4′- or3,3′,4,4′-biphenyltetracarboxylic acid or a 2,3,3′,4′- or3,3′,4,4′-biphenyltetracarboxylic acid salt or esterified derivative.The biphenyltetracarboxylic acid component may also comprise a mixtureof the aforementioned biphenyltetracarboxylic acids.

[0057] As other tetracarboxylic acid components there may be mentionedtetracarboxylic acids such as pyromellitic acid,3,3′,4,4′-benzophenonetetracarboxylic acid,2,2-bis(3,4-dicarboxyphenyl)propane, bis(3,4-dicarboxyphenyl)sulfone,bis(3,4-dicarboxyphenyl)ether, bis(3,4-dicarboxyphenyl)thioether andtheir acid anhydride, salt or ester derivatives. A portion of thesearomatic tetracarboxylic acid components may also be replaced with analiphatic tetracarboxylic acid such as butanetetracarboxylic acid or ananhydride, salt or ester derivative thereof in a proportion of no morethan 10 molar percent and especially no more than 5 molar percent withrespect to the total tetracarboxylic acid component.

[0058] Particularly preferred as tetracarboxylic acid components are3,3′,4,4′-biphenyltetracarboxylic dianhydride, pyromellitic dianhydrideand 3,3′,4,4′-benzophenonetetracarboxylic dianhydride.

[0059] In the first method described above as an example for fabricationof a porous insulating film, a polyimide precursor solution is preparedby dissolving the polyimide precursor in the organic solvent to apolyimide precursor proportion of 0.3-60 wt %, and preferably 1-30 wt %in the solution (the organic solvent may also be added or the polymersolution itself may be used). If the polyimide precursor proportion isless than 0.3 wt % the film strength of the fabricated porous film maybe reduced. If it is greater than 60 wt %, the permeability of theporous film may be lower, and therefore the range given above ispreferred. The solution viscosity of the prepared polyimide precursorsolution is 10-10,000 poise, and preferably 40-3000 poise. If thesolution viscosity is less than 10 poise the film strength of thefabricated porous film may be reduced, and if it is higher than 10,000poise casting into a film may be more difficult, and therefore theranges given above are preferred.

[0060] After the polyimide precursor solution has been cast into a castfilm, it is made into a laminated film by laying a solvent substitutionrate adjustor on at least one side thereof. The method of obtaining thecast laminated film of the polyimide precursor solution is notparticularly restricted, and various techniques may be employed,including a method in which the polyimide precursor solution is castonto a plate of glass or the like serving as the base or onto a movablebelt and the cast surface is covered with a solvent substitution rateadjustor; a method in which a spray or doctor blade method is used tothinly coat the polyimide precursor solution onto a solvent substitutionrate adjustor; and a method in which the polyimide precursor solution isextruded from a T-die and filled between solvent substitution rateadjustors, to obtain a 3-layer laminated film with the solventsubstitution rate adjustor on both sides thereof.

[0061] The solvent substitution rate adjustor is preferably one withpermeability of a level that allows the polyimide precursor solvent andthe solidifying solvent to permeate at a suitable rate when themultilayer film contacts with the solidifying solvent to precipitate thepolyimide precursor. In particular, the gas permeability is preferably50-1000 sec/100 cc, and especially 250-800 sec/100 cc. The filmthickness of the solvent substitution rate adjustor is 5-500 μm andpreferably 10-100 μm, and pores of 0.01-10 μm, and preferably 0.03-1 μm,penetrating in the cross-sectional direction of the film are preferablydistributed at a sufficient density. If the film thickness of thesolvent substitution rate adjustor is lower than the above range thesolvent substitution rate may be too fast, which is unsuitable sincewrinkles may be produced when it contacts with the solidifying solvent,and if it is higher than the above range the solvent substitution ratemay be too slow, thus resulting in a non-uniform pore structure formedinside the polyimide precursor.

[0062] The solvent substitution rate adjustor used may be, specifically,a nonwoven fabric or porous film made from a material of a polyolefinsuch as polyethylene, polypropylene or the like, and the use of apolyolefin fine porous film is particularly preferred for excellentsmoothness of the surface of the fabricated polyimide porous film.

[0063] The multilayered polyimide precursor cast product is contactedwith the solidifying solvent through the solvent substitution rateadjustor, for precipitation of and pore formation in the polyimideprecursor. The solidifying solvent used for the polyimide precursor maybe a non-solvent for the polyimide precursor, for example, an alcoholsuch as ethanol or methanol, or acetone, water or the like, or a mixtureof 99.9-50 wt % of a non-solvent and 0.1-50 wt % of a solvent for thepolyimide precursor. There are no particular restrictions on thecombination for a non-solvent and solvent, but it is preferably one thatgives a uniform porous structure for the precipitated polyimideprecursor when the mixture comprising the non-solvent and solvent isused as the solidifying solvent.

[0064] In the second method described above as an example forfabrication of a porous insulating film with dense layers on both sides,the film-like composition (C) comprises a homogeneous solution of thepolyimide precursor (A) which is a solution preferably comprising 0.3-60wt % and especially 1-30 wt % of the polyimide precursor and 99.7-40 wt% and especially 99-70 wt % of a good solvent for the polyimideprecursor, and a poor solvent (B) for the polyimide precursor, which isa poor solvent having a boiling point or thermal decompositiontemperature at a temperature higher than or equal to the imidationinitiation temperature of the polyimide precursor (usually about 170°C.), wherein the poor solvent used is a compound having a boiling pointor thermal decomposition temperature higher than or equal to theimidation initiation temperature of the polyimide precursor, in order toobtain a film-like composition.

[0065] The poor solvent (B) is preferably a poor solvent having a higherboiling point than the good solvent for the polyimide precursor in thehomogeneous solution (A) of the polyimide precursor. Specifically, it isa poor solvent with a boiling point or thermal decomposition temperatureof 150-500° C., and preferably a boiling point or decompositiontemperature of 170-300° C.

[0066] As the poor solvent (B) there may be mentioned cyclic ethers withboiling points of 100° C. and below.

[0067] The poor solvent (B) for the polyimide precursor is preferablyone that is removed from the film composition (C) by evaporation orthermal decomposition in the step of proceeding heat imidation of thepolyimide precursor. If the poor solvent (B) is not removed from thefilm-like composition (C) in the step of proceeding heat imidation, theresulting porous film may not be suitable for the field of electronicmaterials.

[0068] As poor solvents (B) that satisfy these conditions there may bementioned linear monohydric alcohols with 7 or more aliphatic carbonatoms, dihydric alcohols with 7 or more carbon atoms, branched alcohols,cyclic alcohols, alicyclic alcohols, aromatic alcohols and otherhigh-boiling-point alcohols.

[0069] As specific examples there may be mentioned n-heptanol (boilingpoint: 176.3° C.), n-octanol (boiling point: 179.3° C.), n-nonanol(boiling point: 203° C.), n-decanol (boiling point: 231° C.),n-undecanol (boiling point: 243° C.), 2-octanol (boiling point: 179°C.), 2-ethylhexanol (boiling point: 184.7° C.), 2,6-dimethyl-4-heptanol(boiling point: 178° C.), 3,5,5-trimethyl-1-hexanol (boiling point: 194°C.), tetrahydrofurfuryl alcohol (boiling point: 178° C.), a-terpineol(boiling point: 219° C.), furfuryl alcohol (boiling point: 170° C.) andbenzyl alcohol (boiling point: 205.8° C.). Particularly preferred foruse is n-decanol.

[0070] For ether systems, the poor solvent (B) may be methylanisole(boiling point: 171-177° C.), ethyl benzyl ether (boiling point: 186°C.), diisoamyl ether (boiling point: 173.2° C.), 1,8-cineole (boilingpoint: 176.4° C.), phenetole (boiling point: 170.3° C.), butyl phenylether (boiling point: 210.2° C.) or the like.

[0071] For polyhydric alcohols and their derivatives, the poor solvent(B) may be ethyleneglycol (boiling point: 197° C.), dipropyleneglycol(boiling point: 231.8° C.), dipropyleneglycol monoethylether (boilingpoint: 198° C.), dipropyleneglycol monobutylether (boiling point: 231°C.), triethyleneglycol dimethylether (boiling point: 216° C.),trimethyleneglycol (boiling point: 213° C.), hexyleneglycol (boilingpoint: 213° C.) or the like.

[0072] Particularly preferred for use is ethyleneglycol.

[0073] As cyclic ethers with boiling points below 100° C. that aresuitable as the poor solvent (B) there may be mentioned oxetane (boilingpoint: 48° C.), tetrahydrofuran (boiling point: 66° C.) andtetrahydropyrane (boiling point 88° C.).

[0074] The film-like composition (C) comprising the homogeneous solution(A) of the polyimide precursor and the poor solvent (B) for thepolyimide precursor can be obtained by using either of the followingmethods, method (a) or method (b).

[0075] For method (a), the poor solvent (B) may be added at 10-40 partsby weight and especially 15-30 parts by weight to 100 parts by weight ofthe homogeneous solution (A) of the polyimide precursor to prepare auniform mixed solution, and the mixed solution cast onto a substrate toobtain a film-like composition (C).

[0076] In this method, if the amount of the poor solvent (B) withrespect to 100 parts by weight of the homogeneous solution (A) is lessthan 10 parts by weight the effect of adding the poor solvent will bereduced, while if the amount of the poor solvent (B) is greater than 40parts by weight it will be difficult to achieve a uniform mixedsolution, and therefore the proportions specified above are preferred.The good solvent in the homogeneous solution (A) and the poor solvent(B) may be each of a single type or a mixed solvent of 2 or more types.

[0077] For method (b), the homogeneous solution (A) of the polyimideprecursor may be cast onto a substrate to make a solution film, and thenthis solution film may be immersed in a solidifying bath comprising thepoor solvent (B) to precipitate a film-like product containing the poorsolvent (B) and the film-like product drawn out from the solidifyingbath to obtain a film-like composition (C).

[0078] In the third method described above as an example for fabricationof a porous insulating film, the non-solvent vapor exposure treatmentmay be accomplished by a technique such as a method in which a gascontaining a non-solvent for the polyimide precursor, for example, analcohol such as ethanol or methanol, or acetone, water or the like inthe vapor phase is blown onto the surface of the polyimide precursorsolution to incorporate the non-solvent vapor into the precursorsolution, or a method in which the polyimide precursor solution is heldfor a prescribed period of time in a treatment tank filled with theabove-mentioned vapor, or conveyed through it with a belt, forcondensation of the non-solvent vapor on the precursor solution.

[0079] The non-solvent vapor exposure treatment step is preferablycontinued until condensation of about 0.1 mole or more of thenon-solvent vapor per square meter of the precursor solution surface, inorder to inhibit formation of a dense layer. There is no particularupper limit on the time for which the non-solvent vapor exposuretreatment is continued, but it is preferably terminated just before theincreasing amount of non-solvent condensing on the precursor solutionsurface causes precipitation of the precursor and clouding of theprecursor solution, in order to obtain a homogeneous porous structure.

[0080] The temperature of the precursor solution during the non-solventvapor exposure treatment step may be room temperature, but this is not alimitation so long as conditions are satisfied for condensation of thenon-solvent vapor on the polyimide precursor solution surface.

[0081] The atmosphere of the precursor solution in the non-solvent vaporexposure treatment step may be atmospheric air, but this is also not alimitation so long as conditions are satisfied for condensation of thenon-solvent vapor on the polyimide precursor solution surface.

[0082] The film of the polyimide precursor solution treated in thenon-solvent vapor exposure treatment step is contacted with asolidifying solvent for precipitation of and pore formation in thepolyimide precursor. The solidifying solvent for the polyimide precursormay be a non-solvent for the polyimide precursor, for example, analcohol such as ethanol or methanol, or acetone, water or the like, or amixture of 99.9-50 wt % of a non-solvent and 0.1-50 wt % of a solventfor the polyimide precursor.

[0083] As the method for casting a dope comprising a mixed solution ofthe polyimide precursor homogeneous solution (A) and the poor solvent(B) or the polyimide precursor homogeneous solution (A) onto thesubstrate to form a sheet in the aforementioned third method, there maybe mentioned a method in which coating techniques employing spraying ordoctor blade methods, or techniques of extrusion from a T-die or thelike, and preferred is a method of casting onto a substrate of glass orthe like or onto a movable belt substrate, or a method of extrusion ofthe polyimide precursor solution from a T-die.

[0084] The aforementioned dope solution for casting may also contain asurfactant, flame retardant or coloring agent, or reinforcing materialssuch as glass fibers, silicon-based fibers, inorganic powder and thelike. Such additives and reinforcers may be added to the polyimideprecursor homogeneous solution (A) or to the dope solution for casting.

[0085] The pore-formed polyimide precursor film as the film-likecomposition obtained by any of these three methods is then subjected tothermal imidation treatment or chemical imidation treatment. Thermalimidation of the polyimide precursor porous film is accomplished byfixing the polyimide precursor porous film using a pin, chuck or pinchroll in order to prevent its heat shrinkage, and subjecting it to atemperature of 280-500° C. in air for 5-60 minutes.

[0086] Chemical imidation treatment of the polyimide precursor porousfilm is accomplished by using an aliphatic acid anhydride or an aromaticacid anhydride as a dehydrating agent and a tertiary amine such astriethylamine as a catalyst. Imidazole or benzimidazole, or substitutedderivatives thereof, may also be used as according to JapaneseUnexamined Patent Publication No. 4-339835.

[0087] Chemical imidation treatment of the polyimide precursor porousfilm is preferably used for fabrication of a polyimide porous film witha multilayer construction. A multilayer polyimide porous film may beobtained, for example, by subjecting the surface of a polyolefin fineporous film used as the solvent substitution rate adjustor to plasma,electron beam or chemical treatment for modification of the interfaceadhesion with the polyimide porous layer, and then forming a multilayerstructure with the cast polyimide precursor solution and contacting itwith a solidifying solution to precipitate the cast polyimide precursorsolution and form pores therein, to give a precursor porous film with amultilayer structure. Finally, with chemical imidation treatment it ispossible to fabricate a multilayer polyimide porous film.

[0088] The porous polyimide film fabricated in this manner has aporosity of 15-80%, preferably 30-80% and especially 40-70%, and morepreferably the mean pore size is 0.01-5 μm and especially 0.05-1 μm inthe center and both surfaces, with a maximum pore size of no greaterthan 10 μm, although these will differ somewhat depending on thefabrication conditions that are selected. If the mean pore size is lessthan 0.01 μm it may be difficult to achieve a porosity of 15% orgreater, and if the mean pore size is larger than 5 μm the insulatingmaterial may be undesirable in terms of precision in fine conformationworking and bending.

[0089] The porous polyimide film may have either a single layer ormultilayer construction, while the entire film is adjusted to a filmthickness of 5-150 μm, preferably 5-125 μm and especially 5-100 μm, andthe polyimide porous layer preferably has a heat resistance temperatureof 200° C. or above and a heat shrinkage of no greater than ±1% whenheat treatment is carried out for 8 hours at 105° C.

[0090] According to the invention, a substrate may be constructedcontaining a porous polyimide film which is a porous polyimide filmobtained in the manner described above having a porosity of from 15 vol% to 80 vol %. This can give a low dielectric constant polyimideinsulating film or substrate with a dielectric constant of no greaterthan 2.5. The porosity can also give a dielectric constant of less than2 which cannot be realized with bulk plastics.

[0091] A porosity of less than 15 vol % results in a larger dielectricconstant, and a porosity of greater than 80 vol % results in lowstrength of the substrate, and therefore neither situation is preferred.

[0092] According to the invention, electronic device substrate materialscan be easily obtained having heat resistance temperatures of 200° C. orabove. In such a construction, the presence of the space portion of thepolyimide material containing gas with a much lower dielectric constantthan the solid portion results in a film or substrate with a dielectricconstant that is much lower than the polyimide bulk dielectric constant.

[0093] Since the porous polyimide film of the invention has nonlinearpore structure between one side and the other side even when in contactwith a conductor, it is resistant to phenomena such as corona discharge,and is also resistant to dielectric deterioration due to dielectricbreakdown and the like.

[0094] The porous polyimide film as the porous insulating material ofthe invention has a smooth surface and can also be used by mounting withlamination of one or a plurality of layers of the porous polyimide film,or a different dense polyimide film may be laminated with the porouspolyimide film. For example, the porous polyimide film may be laminatedonto an organic, inorganic or metal substrate such as a polyimide film,silicon substrate, glass substrate, carbon substrate or aluminumsubstrate, either directly or via a heat resistant adhesive.

[0095] In addition, a heat resistant, film-like adhesive layer such as athermoplastic polyimide or polyimidosiloxane-epoxy resin may also belaminated on either or both sides of the porous polyimide film servingas the porous insulating material of the invention, and a protectivefilm comprising a resin film of an aromatic polyimide, aromaticpolyester, polyethylene, polypropylene, 1-polybutene, etc. may also beprovided thereon to obtain the laminate.

[0096] This laminate prevents adhesion of dust and thus facilitatestransport, and the protective film can be peeled off at the time of useand laminated onto a known conductive metal electronic circuit foil suchas an electrolytic copper foil, rolled copper foil, rolled aluminum foilor the like to easily obtain a circuit substrate.

[0097] A heat resistant film-like adhesive layer may also be laminatedonto either or both sides of the porous polyimide film serving as theporous insulating material of the invention, and a conductive metalelectronic circuit foil laminated thereover to prepare the laminate.

[0098] Alternatively, the pore-formed polyimide precursor porous filmmay be combined with one side of the conductive metal electronic circuitfoil and then heated to dryness for complete imidation to obtain thelaminate.

[0099] Also, one side of a porous polyimide film serving as the porousinsulating material of the invention may be combined with one side of apolyimide film, an inorganic substrate such as a silicon substrate,glass substrate or carbon substrate or a metal substrate such as analuminum substrate, with a heat resistant film-like adhesive layersandwiched between them, and then subjected to thermocompressionbonding, after which the other side of the porous polyimide film servingas the porous insulating material of the laminate may be combined with aconductive metal foil with a heat resistant film-like adhesive layersandwiched between them and then subjected to thermocompression bondingto obtain a laminated substrate as the laminate.

[0100] Also, an inorganic, organic or metal substrate such as a siliconsubstrate may be provided on one side of the porous polyimide filmserving as the porous insulating material via a heat resistant adhesiveand a dense polyimide layer on the other side either directly or via aheat resistant adhesive, with a conductive metal circuit layer furtherprovided thereover, to prepare the laminate. In this case, a metal foilmay be used as the conductive metal circuit layer.

[0101] Alternatively, a known metal such as copper, nickel, chromium,aluminum or the like may be used on one or both sides of the porouspolyimide film serving as the porous insulating material to form aconductive metal circuit layer by a combination of vapor deposition(vacuum deposition or sputtering) and plating (electroless plating orelectroplating).

[0102] The porous polyimide film serving as the porous insulatingmaterial of the invention may also be used after vacuum and/or heatdrying to remove any moisture included in the continuous pores due tothe surroundings.

[0103] The porous insulating film having a mean pore size of 0.01-5 μm,a porosity of 30-80%, a maximum pore size of no greater than 10 μm, afilm thickness of 5-100 μm, a resistance to passage of air of from 30sec/100 cc to 2000 sec/100 cc, a heat resistance temperature of 200° C.or above and a heat shrinkage of no greater than ±1%, with finecontinuous pores reaching to both surfaces, exhibits satisfactory heatresistance and electrical insulating properties as well as ion mobility,and is therefore suitable for such uses as battery separators and thelike.

[0104] Examples of the invention will now be provided.

[0105] The measurement methods used are explained below.

[0106] (1) Porosity

[0107] The film thickness and weight of a porous film cut to aprescribed size were measured, and the porosity was calculated based onthe weight according to the equation given below. In this equation, S isthe porous film area, d is the film thickness, W is the measured weightand D is the polyimide density; the polyimide density was assumed to be1.34 g/m³.

Porosity (%)=100−100×(W/D)/(S×d)

[0108] (2) Dense layer thickness, proportion

[0109] A cross-section was cut out of the porous film, the dense layerthickness and entire film thickness were measured with a scanningmicroscope, and the dense layer proportion was calculated.

[0110] (3) Pore size

[0111] A cross-section was cut out of the porous film and measured witha scanning microscope.

[0112] (4) Dielectric constant

[0113] This was measured at a frequency of 1000 Hz, according toJIS-C-6481.

[0114] (5) Tensile strength

[0115] This was measured according to JIS K7127. A Tensilon UniversalTester (product of Toyo Baldwin Co.) was used for measurement at astrain rate of 10 mm/min.

[0116] (6) Shrinkage factor

[0117] A sample with graduated markings of a specified length wasallowed to stand in an unbound condition for 8 hours in an oven set to150° C., and after being removed its dimensions were measured. The heatshrinkage factor was determined by the equation given below. In theequation, L₁ represents the film dimensions after being removed from theoven, and Lo represents the initial film dimensions.

Heat shrinkage factor (%)=[1−(L₁/L₀)]=100

[0118] (7) Penetrating strength

[0119] A sample was fixed in a round-hole holder with a diameter of11.28 mm and a surface area of 1 cm², and then a 1 mm diameter needlewith a point shape of 0.5 R was lowered at a speed of 2 mm/sec untilpenetration and the penetration load was measured.

[0120] (8) Heat resistance temperature

[0121] A nonaqueous electrolyte solution was filled into the fine poresand the temperature dependence of the electrical resistance was measuredusing a measuring device by Hioki E.E. Corp. (3520 LCR HiTESTER), whilethe porous film structure was observed to determine whether or not adeformation occurred, and the highest temperature at which no suchproblems were observed was recorded as the heat resistance temperature.

[0122] (9) Resistance to passage of air

[0123] This was measured using a Gurley densometer according toJIS-P-8117.

EXAMPLE 1

[0124] Using s-BPDA as the tetracarboxylic acid component and DADE asthe diamine component, these were dissolved in NMP with the DADE at amolar ratio of 0.994 with respect to the s-BPDA and the monomercomponents at a total weight of 10 wt %, and was polymerized at atemperature of 40° C. for 6 hours to obtain a polyimide precursor. Thesolution viscosity of the polyimide precursor solution was 500 poises.

[0125] The resulting polyimide precursor solution was cast onto a glasssubstrate to a thickness of about 75 μm, and then the surface thereofwas covered with a polyolefin fine porous film (product of Ube Kosan,KK.) having a resistance to passage of air of 550 sec/100 cc as asolvent substitution rate adjustor, without producing wrinkles. Thelaminate was immersed in methanol for 5 minutes and subjected to solventsubstitution with the solvent substitution rate adjustor forprecipitation of and pore formation in the polyimide precursor.

[0126] The precipitated polyimide precursor porous film was immersed inwater for 15 minutes and then released from the glass substrate andsolvent substitution rate adjustor and affixed to a pin tenter for heattreatment in air at 300° C. for 10 minutes, to obtain a porous polyimidefilm.

[0127] The resulting porous polyimide film was observed with a scanningelectron microscope and found to have fine curved continuous pores, athickness of 40 μm, an average pore size of 0.5 μm and a maximum poresize of no greater than 10 μm on the surface, an average pore size inthe range of 0.01-2 μm and a maximum pore size of no greater than 10 μmat the center, a porosity of 60%, a resistance to passage of air of 180sec/100 cc, a penetrating strength of 350 gf, a heat shrinkage of 0.4%and a heat resistance temperature of 200° C. or above. Scanning electronmicroscope photographs are shown in FIGS. 7 and 8.

EXAMPLE 2

[0128] Using s-BPDA as the tetracarboxylic acid component and p-PDA asthe diamine component, these were dissolved in NMP with the p-PDA at amolar ratio of 0.994 with respect to the s-BPDA and the monomercomponents at a total weight of 15 wt %, and polymerization wasconducted at a temperature of 40° C. for 6 hours to obtain a polyimideprecursor. The solution viscosity of the polyimide precursor solutionwas 500 poise.

[0129] The resulting polyimide precursor solution was cast onto a glasssubstrate to a thickness of about 150 μm, and then the surface thereofwas covered with a polyolefin fine porous film (product of Ube Kosan,KK.) having a gas permeability of 550 sec/100 cc as a solventsubstitution rate adjustor, without producing wrinkles. The laminate wasimmersed in methanol for 5 minutes and subjected to solvent substitutionwith the solvent substitution rate adjustor for precipitation of andpore formation in the polyimide precursor.

[0130] The precipitated polyimide precursor porous film was immersed inwater for 15 minutes and then released from the glass substrate andsolvent substitution rate adjustor and affixed to a pin tenter for heattreatment in air at 400° C. for 10 minutes, to obtain a porous polyimidefilm.

[0131] Observation with a scanning electron microscope revealed finecurved continuous pores, a thickness of 50 μm, an average pore size of0.5 μm and a maximum pore size of no greater than 10 μm on the surface,an average pore size in the range of 0.01-2 μm and a maximum pore sizeof no greater than 10 μm at the center, a porosity of 60%, a gaspermeability of 380 sec/100 cc, a penetrating strength of 520 gf, a heatshrinkage of 0.3% and a heat resistance temperature of 200° C. or above.

EXAMPLE 3

[0132] Using s-BPDA as the tetracarboxylic acid component and apreparation of p-PDA and DADE in a molar ratio of 85:15 as the diaminecomponent, these were dissolved in NMP with the diamine component at amolar ratio of 0.994 with respect to the s-BPDA and the monomercomponents at a total weight of 15 wt %, and was polymerized at atemperature of 40° C. for 6 hours to obtain a polyimide precursor. Thesolution viscosity of the polyimide precursor solution was 600 poises.

[0133] The resulting polyimide precursor solution was cast onto a glasssubstrate to a thickness of about 150 μm, and then the surface thereofwas covered with a polyolefin fine porous film (product of Ube Kosan,KK.) having a resistance to passage of air of 550 sec/100 cc as asolvent substitution rate adjustor, without producing wrinkles. Thelaminate was immersed in methanol for 5 minutes and subjected to solventsubstitution with the solvent substitution rate adjustor forprecipitation of and pore formation in the polyimide precursor.

[0134] The porous polyimide precursor film was immersed in water for 15minutes and then released from the glass substrate and solventsubstitution rate adjustor and affixed to a pin tenter for heattreatment in air at 330° C. for 10 minutes, to obtain a porous polyimidefilm.

[0135] Observation with a scanning electron microscope revealed finecurved continuous pores, a thickness of 65 μm, an average pore size of0.9 μm and a maximum pore size of no greater than 10 μm on the surface,an average pore size in the range of 0.01-2 μm and a maximum pore sizeof no greater than 10 μm at the center, a porosity of 63%, a resistanceto passage of air of 580 sec/100 cc, a penetrating strength of 860 gf, aheat shrinkage of 0.2% and a heat resistance temperature of 200° C. orabove.

EXAMPLE 4

[0136] A porous polyimide film was obtained in the same manner asExample 1, except that the thickness of the cast polyimide precursorsolution in Example 1 was changed to about 150 μm.

[0137] Observation with a scanning electron microscope revealed finecurved continuous pores, a thickness of 84 μm, an average pore size of0.4 μm and a maximum pore size of no greater than 10 μm on the surface,an average pore size in the range of 0.01-2 μm and a maximum pore sizeof no greater than 10 μm at the center, a porosity of 60%, a resistanceto passage of air of 270 sec/100 cc, a penetrating strength of 440 gf, aheat shrinkage of 0.3% and a heat resistance temperature of 200° C. orabove.

[0138] The dielectric constant (ε) and dielectric loss tangent (tanδ) ofthe porous polyimide film were measured under the conditions describedbelow. The measuring electrode was 200 nm, 8 mm aluminum, the measuringapparatus was a Hewlett-Packard 4194A, the measuring frequency range wasfrom 1 kHz to 10 MHz, and the average value of 16 measurements for eachmeasuring point was used. The measuring temperature was 25° C. Themeasuring results for the measuring frequencies of 1000 Hz and 10 MHzare given below. The dielectric constant was virtually unchanged withinthis measuring range.

[0139] 1000 Hz

[0140] Dielectric constant: 1.68

[0141] Loss tangent: 0.0025

[0142] 10 MHz

[0143] Dielectric constant: 1.67

[0144] Loss tangent: 0.0053

[0145] The dielectric constant of the dense polyimide (bulk polyimide)was 3.2 to 3.4 at each frequency, indicating the properties of theporous polyimide film of the invention.

EXAMPLE 5

[0146] A porous polyimide film was obtained in the same manner asExample 2, except that the thickness of the cast polyimide precursorsolution was changed to about 200 μm.

[0147] Observation with a scanning electron microscope revealed finenonlinear pores, a thickness of 62 μm, an average pore size of 0.7 μmand a maximum pore size of no greater than 10 μm on the surface, anaverage pore size in the range of 0.01-2 μm and a maximum pore size ofno greater than 10 μm at the center, a porosity of 64% and a heatresistance temperature of 200° C. or above.

[0148] The dielectric constant (ε) and dielectric loss tangent (tanδ)were as shown below. The dielectric constant was virtually unchangedwithin the measuring range.

[0149] 1000 Hz

[0150] Dielectric constant: 1.73

[0151] Loss tangent: 0.0029

[0152] 10 MHz

[0153] Dielectric constant: 1.71

[0154] Loss tangent: 0.0062

EXAMPLE 6

[0155] A porous polyimide film was obtained in the same manner asExample 3, except that the thickness of the cast polyimide precursorsolution was changed to about 150 μm.

[0156] Observation with a scanning electron microscope revealed finenonlinear continuous pores, a thickness of 51 μm, an average pore sizeof 0.9 μm and a maximum pore size of no greater than 10 μm on thesurface, an average pore size in the range of 0.01-2 μm and a maximumpore size of no greater than 10 μm at the center, a porosity of 43% anda heat resistance temperature of 200° C. or above.

[0157] The dielectric constant (ε) and dielectric loss tangent (tanδ)were as shown below.

[0158] The dielectric constant was virtually unchanged within themeasuring range.

[0159] 1000 Hz

[0160] Dielectric constant: 2.34

[0161] Loss tangent: 0.0032

[0162] 10 MHz

[0163] Dielectric constant: 2.31

[0164] Loss tangent: 0.0055

[0165] Since dropping methanol onto one side of the porous polyimidefilms obtained in Examples 1-6 and allowing it to remain resulted inwetting of the other side, the formation of continuous pores wasconfirmed. Separately there were fabricated a dense polyimide film andan independent pore polyimide film, and dropping of methanol in the samemanner resulted in no wetting of the other side.

EXAMPLE 7

[0166] Each of the porous polyimide films obtained in Examples 4, 5 and6 and an electrolytic copper foil (35 μm) or rolled copper foil (10 μm)were combined using a 20-μm thick film-like polyimidosiloxane-epoxyresin-based thermosetting heat resistant adhesive (UPA, product of UbeKousan, KK.), and these were subjected to heat treatment under anitrogen gas stream at 100° C. for one hour, 120° C. for one hour and180° C. for 5 hours to harden the adhesive and obtain a firmly adheredlaminate substrate. The adhesive strength was at least 1.0 kg/cm for allthe films.

[0167] The dielectric constant values for separately prepared laminatesof porous polyimide films and the adhesive were as follows.

[0168] 1000 Hz

[0169] Dielectric constant: 1.8, 1.8 and 2.4

[0170] 10 MHz

[0171] Dielectric constant: 1.8, 1.8 and 2.4

EXAMPLE 8

[0172] Using s-BPDA as the tetracarboxylic acid component and DADE asthe diamine component, these were dissolved in NMP (bp: 202° C.) withthe DADE at a molar ratio of 0.994 with respect to the s-BPDA and themonomer components at a total weight of 18 wt %, and polymerization wasconducted at a temperature of 40° C. for 6 hours to obtain a homogeneoussolution (A) of the polyimide precursor.

[0173] Next, n-decanol (bp: 231° C.) was added as a poor solvent (B) tothe homogeneous solution (A) of the polyimide precursor, to obtain adope solution containing the polyimide precursor at about 15 wt %, NMPat 68 wt % and n-decanol at 17 wt %.

[0174] The solution was cast onto a glass substrate to a thickness ofabout 150 μm and dried at about 80° C. The polyimide precursor film(polyimide precursor gel) that separated from the glass substrate at thestart of drying was affixed to a pin tenter for heat treatment in air at300° C. for 40 minutes, to obtain a porous polyimide film.

[0175] A cross section of the resulting porous polyimide film wasobserved with a scanning microscope, and both surface layers wereconfirmed to be dense layers with fine curved continuous pores at thecenter in the cross-sectional direction of the film. Measurement of thetensile strength, film thickness, porosity, surface condition, denselayer thickness, center condition, heat shrinkage ratio and penetratingstrength of the porous polyimide film gave the following results.

[0176] Evaluation results

[0177] Tensile strength: 460 kgf/cm²

[0178] Film thickness: 85 μm

[0179] Porosity: 65%

[0180] Surface condition: dense layer

[0181] Dense layer thickness: 2.1 μm (both sides)

[0182] Center condition: Porous layer: mean pore size in

[0183] range of 0.01-2 μm, maximum pore size of 10 μm

[0184] Heat shrinkage: 0.3%

[0185] Penetrating strength: 385 gf

EXAMPLE 9

[0186] A porous polyimide film was obtained in the same manner asExample 8, except that ethyleneglycol (bp: 197.8° C.) was added to thehomogeneous solution (A) of the polyimide precursor as the poor solvent(B) instead of n-decanol, and a dope solution prepared with thepolyimide precursor at about 10 wt %, NMP at 48 wt % and ethyleneglycolat 42 wt % was used.

[0187] A cross-section of the resulting porous polyimide film wasobserved with a scanning microscope, and revealed to have the samestructure and tensile strength as the film of Example 8. Measurement ofthe film thickness, porosity, surface condition, dense layer thickness,center condition, heat shrinkage ratio and penetrating strength gave thefollowing results.

[0188] Evaluation results

[0189] Film thickness: 83 μm

[0190] Porosity: 52%

[0191] Surface condition: dense layer

[0192] Dense layer thickness: 3.0 μm (both sides)

[0193] Center condition: Porous layer: mean pore size in

[0194] range of 0.01-2 μm, maximum pore size of 10 μm

[0195] Heat shrinkage: 0.3%

[0196] Penetrating strength: 408 gf

EXAMPLE 10

[0197] Using ethyleneglycol (bp: 197.8° C.) as the poor solvent (B), ahomogeneous solution (A) of the polyimide precursor was cast onto aglass substrate to make a solution film, which was then immersed into anethyleneglycol solidifying bath together with the glass substrate toprecipitate a film, and the precipitated film was removed from thesolidifying bath and affixed to a pin tenter for immediate heating at300° C. for 40 minutes for imidation to obtain a porous polyimide film.

[0198] A cross section of the resulting porous polyimide film wasobserved with a scanning microscope, and revealed to have the samestructure and tensile strength as the film of Example 8.

[0199] Measurement of the film thickness, porosity, surface condition,dense layer thickness, center condition, heat shrinkage ratio andpenetrating strength gave the following results.

[0200] Evaluation results

[0201] Film thickness: 84 μm

[0202] Porosity: 49%

[0203] Surface condition: dense layer

[0204] Dense layer thickness: 1.8 μm (both sides)

[0205] Center condition: Porous layer: mean pore size in

[0206] range of 0.01-2 μm, maximum pore size of 10 μm

[0207] Heat shrinkage: 0.3%

[0208] Penetrating strength: 410 gf

COMPARATIVE EXAMPLES 1 AND 2

[0209] The same procedure was followed as in Example 8, except thatethanol (bp: 78.3° C.) was used (Comparative Example 1) or methanol (bp:64.1° C.) was used (Comparative Example 2) as the poor solvent (B)instead of n-decanol. Observation of cross-sections of the resultingpolyimide films with a scanning microscope revealed that both were filmsof only a dense layer with no porous layer.

EXAMPLE 11

[0210] A porous polyimide film was obtained by the same procedure as inExample 8, except that the homogeneous solution (A) of the polyimideprecursor was obtained using PPD as the diamine component instead ofDADE, and the heating temperature (maximum temperature) was changed to425° C. The evaluation results for the porous polyimide film were thesame as in Example 8.

EXAMPLE 12

[0211] The porous polyimide films obtained in Examples 8-10 were usedfor evaluation as insulating materials. All exhibited satisfactoryproperties. The porous polyimide film obtained in Example 8 had adielectric constant of 2.1 at 23° C., 1000 Hz and a dielectric loss of0.004.

[0212] Cross sections of each of the porous polyimide films obtained inthe examples (Examples 1-11) were observed under a scanning microscopeand found to be smooth on both surfaces.

COMPARATIVE EXAMPLE 3

[0213] A commercially available non-porous polyimide film with athickness of 25 μm was used for evaluation. It had a dielectric constantof 3.2 at 23° C., 1000 Hz and a dielectric loss of 0.004, and thereforethe dielectric constant was inadequate.

[0214] The present invention, having the construction described above,thus exhibits the following effects.

[0215] According to the invention it is possible to obtain a porousinsulating material with low dielectric constant that cannot be achievedwith bulk polyimides, as well as heat resistance and strong adhesion oradhesion with metals and metal foils.

What is claimed is:
 1. A porous insulating film comprising a highly heatresistant resin film having a fine porous structure with a mean poresize of 0.01-5 μm in at least the center of the film, and having aporosity of 15-80%.
 2. A porous insulating film according to claim 1,wherein the mean pore size is 0.05-1 μm.
 3. A porous insulating filmaccording to claim 1, wherein the porosity is 30-80%.
 4. A porousinsulating film according to claim 1, which has a thickness of 5-150 μm.5. A porous insulating film according to claim 1, wherein the fineporous structure consists of fine continuous pores.
 6. A porousinsulating film according to claim 1, which is fabricated by a filmcasting method.
 7. A porous insulating film according to claim 1,wherein the dielectric constant is no greater than 2.5.
 8. A porousinsulating film according to claim 1, wherein the highly heat resistantresin film is a polyimide film.
 9. A porous insulating film according toclaim 1, wherein the porous structure has fine continuous pores reachingto both surfaces.
 10. A porous insulating film according to claim 9,which has a porosity of 30-80%, a maximum pore size of no greater than10 μm, a film thickness of 5-100 m, a resistance to passage of air offrom 30 sec/100 cc to 2000 sec/100 cc, a heat resistance temperature of200° C. or above and a heat shrinkage of no greater than ±1%.
 11. Aporous insulating film according to claim 1, wherein the porousstructure has a dense layer on both surfaces of the film.
 12. A laminateprepared by forming a heat resistant adhesive layer on one or both sidesof a porous insulating film according to claim
 1. 13. A laminateprepared by laminating a conductive metal layer for an electroniccircuit on one or both sides of a porous insulating film according toclaim 1, either directly or via a heat resistant adhesive layer.
 14. Alaminate prepared by laminating an inorganic or metal substrate onto oneside of a porous insulating film according to claim 1 and a conductivemetal layer onto the other side, each via a heat resistant adhesivelayer.